This invention relates to a target structure for use in a photoconductive image pickup tube and more particularly to a target structure including a heterojunction and utilized in a vidicon or photoconductive image pickup tube and a method of manufacturing the same.
As an image pickup tube including a target which utilizes a non-crystalline photoconductive film, a vidicon has been known which includes an ohmic junction utilizing a film of antimony trisulfide.
Recently, an image pickup tube including a photoconductive target which utilizes a non-crystalline photoconductive film wherein use is made of a heterojunction between a P-type photoconductive film containing selenium and an intensifier such as tellurium, and an N-type conductive film has been proposed.
The image pickup tube of this type is characterized in that it has a wide range of spectrum sensitivity, fast response time, low dark current and a high resolution, and that it is easy to manufacture.
Typically, the target structure of the image pickup tube having these characteristics is constructed such that a transparent conductive film consisting essentially of indium oxide or stannic oxide having N-type conductivity is coated on the rear surface of a glass substrate or a glass window that transmits the incident light rays to the image pickup tube and that a P-type photoconductive film comprising selenium, less than 30 atomic % of tellurium, and less than 30 atomic % of arsenic, for example, a P-type photoconductive film comprising a mixture of a first photoconductive substance consisting of selenium and less than 40 atomic % of tellunium and a second photoconductive substance consisting of selenium and 10 atomic % of arsenic is deposited on the rear surface of the N-type transparent conductive film through a heterojunction surface.
According to another type, an N-type transparent semiconductor film is formed on the rear side of said N-type transparent conductive film by the vapour deposition of cadmium selenide, cadmium sulfide, zinc sulfide, gallium arsenic, germanium or silicon and said P-type photoconductive film is formed on the rear surface of the N-type transparent semiconductive film through a heterojunction surface. Furthermore, for the purpose of improving the landing characteristic of an electron beam emitted from an electron beam emitting device on the photoconductive film a porous film of antimony trisulfide (Sb.sub.2 S.sub.3) is formed on the rear surface of the P-type photoconductive film. In these cases, as will be described later with reference to the accompanying drawings the tellurium in the first photoconductive substance presents throughout the thickness of the P-type photoconductive film and the concentration of the tellurium increases substantially continuously from the heterojunction surface whereas the concentration of the arsenic in the second photoconductive substance is substantially uniform from the heterojunction surface to the P-type photoconductive film and throughout the thickness thereof.
With this construction, the region in which the concentration of tellurium is high and hence having an extremely low specific resistance is located close to the heterojunction surface so that the heterojunction surface is deteriorated and the initial dark current characteristic is greatly impaired. Where the target is stored or left standstill in atmosphere at a temperature higher than 60.degree. C the heterojunction surface is deteriorated to increase the dark current due to a slight diffusion of tellurium. Such variation in the dark current characteristic causes a poor colour balance of a picture picked up by the image pickup tube thus degrading the quality of the picture.
Moreover, since tellurium has a larger tendency of crystallization under heat than selenium, it hastens crystallization of the P-type photoconductive film thus causing local decrease of the film resistance. As a result, defects in the form of white spots are formed in the picture thereby greatly decreasing the quality of the picture.